Our aim is to advance the understanding of self renewal and differentiation of hemopoietic pluripotential stem cells by studying marrow transfusions. We shall use irradiated recipients as well as normal recipient which have recently been found to allow proliferation of transfused stem cells. The fate of the transfused marrow cells will be followed by use of a special strain of mice in which individual animals have one of 2 variants of an enzyme (phosphoglycerate kinase). By selecting mice with one variant as donors, the others as recipient, the electrophoretically distinguishable enzymes identify groups of cells as either donor or recipient in origin. Specifically (1) we shall study the failure of marrow to repopulate an irradiated mouse after 4 or 5 successful serial transfusions into irradiated recipient. We shall study in particular the role played by transfusion itself. (2) We shall culture stem cells and their progeny after marrow transfusion to gauge the relative role of replication and differentiation. (3) We shall investigate whether the reported increase in seeding of marrow cells after certain pretreatments leads to effective augmentation of proliferation of donor marrow in normals. (4) To investigate whether the rejection of female marrow by male hosts which is abolished by irradiation of the host is an immunologic phenomenon as has been suggested and whether the rejection can be restored by transfusion of immunocompetent cell.